Copied from the upstream bug [1]:

"This bug report is related to Security.

Currently novnc is allowing open direction, which could potentially be used for phishing attempts

To test.
https://<sites' vnc domain>//example.com/%2F..
include .. at the end

For example:
http://vncproxy.my.domain.com//example.com/%2F..

It will redirect to example.com. You can replace example.com with some legitimate domain or spoofed domain.

The description of the risk is
By modifying untrusted URL input to a malicious site, an attacker may successfully launch a phishing scam and steal user credentials.
Because the server name in the modified link is identical to the original site, phishing attempts may have a more trustworthy appearance."

[1] https://bugs.launchpad.net/nova/+bug/1927677